Bone marrow hematopoietic stem cell (HSC) transplantation has been used for the treatment of malignant hematopoietic diseases. However, the aspiration of HSCs from the bone marrow is painful with the potential risk of infection and morbidity, and optimal HSC numbers are not always available for successful transplantation in many cases. To avoid these disadvantages, HSCs from the human umbilical cord blood (UCB) have been successfully used for the treatment of both malignant and non-malignant hematopoietic diseases in children in autologous and allogeneic settings. UCB contains HSCs and hematopoietic progenitor cells (HPCs) that appear to have higher proliferation rates and immunological tolerance compared to those in bone marrow. Unfortunately, UCB also has its limitations in that the HSC and HPC yields can be low and the cell numbers adequate for the treatment of hematopoietic diseases in children but not adults. It is estimated that for successful engraftment, at least 2.5×106 CD34+ cells per kg of patient body weight is required but a good UCB harvest from a single umbilical cord generates only about 10×106 CD34+ cells which is adequate for only a 4 kg child.
Several approaches have been suggested to alleviate the problems of inadequacy of HSC numbers for transplantation. These include (a) administration of a second UCB unit to the patient from another donor or (b) ex vivo expansion of the same patient's UCBHSCs. The latter approach is more favorable for immunological reasons as cell rejection can be avoided when autologous HSCs are administered. However, any expansion protocol must attempt to simulate as close as possible in vivo hematopoiesis while maintaining the stemness properties of the HSCs.
Human mesenchymal stem cells (MSCs) have been successfully used in vitro as a scaffold for stromal support and expansion of HSCs via cell-to-cell contact. This concept was developed based on the understanding that MSCs exist within the bone marrow in vivo and act as a natural scaffold on which the in vivo HSCs interact and proliferate. It is not definitely known whether the mechanism behind the HSC-MSC interaction that results in HSC proliferation is mediated by diffusible factors crossing over from the MSCs to HSCs during cell-to-cell contact or through secretions by the MSCs into the immediate microenvironment of the HSCs. To avoid immunological complications, autologous MSCs from the same patient's bone marrow or UCB have been used as a scaffold in vitro, for expansion of her own HSCs with successful results. However, the use of MSCs from UCB for stromal support of autologous HSCs has its own limitations in that the numbers of MSCs in UCB are extremely low and their existence in UCB has been controversial. Musina et al., (Bull Exp Biol Med, 143:127-131 (2007)) reported very low counts of UCB MSCs per volume of UCB and showed that such MSCs had low proliferation rates.
Thus, a need exists for improved methods of obtaining HSCs.